Harmonic oscillators known in the art, and implemented in an integrated circuit chip, comprise an inductor and a capacitor, generally known as a tank, operating at a resonance frequency of the tank. Typically, such an oscillator injects a pulse waveform into the tank, which filters out higher current harmonics and generates a sinusoidal voltage waveform at its output. The tank comprises the inductor and capacitor coupled in parallel, and operates in a parallel resonance mode, where the parallel impedance, that is, the impedance of the inductor and capacitor coupled in parallel, is high, generating a relatively high oscillation voltage from a relatively low bias current.
In some applications, for example, in wireless communication apparatus, an oscillator is required that has an extremely low phase noise in combination with a low power consumption. Such a combination is difficult to achieve, particularly if the available power supply voltage Vdd is low, as is often the case with present-day nanometre complementary metal oxide semiconductor (CMOS) processes. Increasing the oscillation voltage swing can reduce the phase noise of an oscillator. However, conventional oscillators are limited by the maximum voltage swing they can provide, which ranges from a peak single-ended voltage of 2Vdd, to 3Vdd, the latter being possible in so-called class-D oscillators. Reducing the inductance of the inductor and increasing the capacitance of the capacitor can also decrease the phase noise. However, if the required inductance is very small, for example, a few tens of picoHenrys, this approach can become difficult to manage due to parasitic inductances and resistances of the integrated circuit that start playing a dominant role. Furthermore, the quality factor of very small inductors is lower than for larger inductors, resulting in a higher power consumption for a given phase noise level.
FIG. 1 illustrates a typical Clapp oscillator employing a parallel resonance mode. Referring to FIG. 1, the Clapp oscillator has a first tank TA comprising a first inductor LA and a first capacitor CA. The first inductor LA and the first capacitor CA are coupled in series to a drain of a first transistor QA. For providing a differential tank voltage VOUT, the Clapp oscillator also has a second tank TB comprising a second inductor LB and second capacitor CB. The second inductor LB and the second capacitor CB are coupled in series to a drain of a second transistor QB. The first and second transistors QA, QB have theirs gates biased by a constant bias voltage VDC. The Clapp oscillator is a current-mode oscillator, which means that the first and second transistors QA, QB operate as transconductors, providing voltage-to-current conversion, and delivering a large current to their respective first and second tanks TA, TB without loading the tanks. Therefore, each transconductor must have high parallel impedance. Although the first and second tanks TA, TB have series coupled inductors and capacitors, the Clapp oscillator does not oscillate at the series resonance frequency of the series coupled inductors and capacitors. Instead, the Clapp oscillator oscillates at a frequency that is determined by all reactive components in the tanks, including the capacitances between the drain and source, and the source and ground, of the first and second transistors QA, QB. These capacitances are also represented in FIG. 1. For a given bias current supplied to sources of the first and second transistors QA, QB, the oscillation amplitude is proportional to the bias current and an equivalent parallel tank resistance. Therefore, for the current-mode Clapp oscillator with a bias current IBIAS provided by first and second current sources IA, IB illustrated in FIG. 1, the amplitude of the tank voltage VOUT can be expressed asVOUT=k·IBIAS·RPEQ  (1)
where RPEQ is the equivalent parallel resistance of each of the tanks, which is proportional to the quality factor Q of each of the tanks, and k is a proportionality factor. In the Clapp oscillator, the parallel resistance of each of the tanks is deteriorated by the feedback at the transistor source through the capacitive tap between drain and source and source and ground.
There is a requirement for an improved oscillator.